The present invention relates to a positive-working photoresist composition or, more particularly, to a positive-working photoresist composition which can be used in the manufacture of finely patterned semiconductor devices such as integrated circuits by a photolithographic process advantageously in respect of the excellent storage stability and highly orthogonal cross sectional profile of the resist patterns formed therewith as well as absence of drawbacks due to precipitation of the photosensitive ingredient contained therein.
As is well known, the progress of the semiconductor technology in recent years is so remarkable, in particular, in the more and more increasing degree of integration in semiconductor-based integrated circuits already running in the era of megabits integration. The photolithographic process is a key process for patterning on semiconductor substrates by virtue of the applicability thereof to any large-scale mass production throughout the history of development of integrated circuits beginning with the dawning stage where the integrated circuits were of the planar type of only a very low degree of integration to the up-to-date stage where the major current of the technology is for VLSIs of megabits integration.
The above mentioned photolithographic process is performed, in principle, by coating the surface of a substrate such as semiconductor silicon wafer with a solution-type photoresist composition followed by drying to provide a uniform photoresist layer on the substrate surface and then pattern-wise exposure of the photoresist layer to light through a photomask bearing a desired circuit pattern to form a latent image followed by development of the latent image to form a patterned photoresist layer on the substrate surface which serves as a mask in the subsequent etching or other processing treatments.
Photoresist compositions used as a masking material against etching can be classified according to the types of the photosensitivity into so-called negative-working and positive-working ones. Namely, the photoresist composition of the former type is insolubilized in a developer solution by exposure to light as a result of a photochemical crosslinking reaction of the polymeric ingredient therein while the photoresist composition of the latter type is imparted with increased solubility in a developer solution by exposure to light. Integrated circuits at the early stage of development were manufactured mostly by using a negative-working photoresist composition of which the photoresist layer was pattern-wise exposed to light through a photomask in direct contact with the photoresist layer. Negative-working photoresist compositions in general have some disadvantages that the resolving power of patterning therewith has a limitation due to swelling of the patterned resist layer in the developer solution in addition to the problem that the photomask used in contacting exposure on the photoresist layer is unavoidably subject to mechanical damages more or less. Accordingly, it is a trend in recent years when the degree of integration in VLSIs is 256 kilobits or larger with a patterning fineness of 1.3 to 1.2 .mu.m or even finer that a positive-working photoresist composition capable of giving a higher resolving power is used in combination with the patterning method by manifying projection exposure to light by more and more replacing the negative-working photoresist compositions. As to the etching treatment with the patterned resist layer as the mask, the wet etching process using an etching solution can no longer be used when the patterning fineness is in the submicron range due to the isotropic nature of the etching therewith. Instead, the etching treatment is performed by the dry etching method using an etchant gas in which etching proceeds anisotropically in the direction perpendicular to the substrate surface resulting in high fidelity to the pattern of the resist layer.
It is a notable fact that the fidelity or orthogonality of the dry etching treatment with a patterned resist layer as a mask is under a profound influence of the cross-sectional profile configuration of the patterned resist layer. In the patterning of the photoresist layer by pattern-wise exposure to light, furthermore, the contrast between the exposed areas and unexposed areas should desirably be as high as possible while the contrast in the manifying projection exposure is disadvantageously low especially when the patterning fineness is in the submicron order. When the exposure dose given on to a positive-working photoresist layer is increased to such an extent that the layer in the exposed areas is imparted with full solubility in a developer solution, the unavoidable exposure dose on the "unexposed" areas or areas not to be exposed must be correspondingly increased to undesirably enhance the solubility of the resist layer in the unexposed areas so that disadvantages are caused in respect of a decrease in the orthogonality of the cross-sectional profile of the patterned resist layer and in the fidelity or dimensional accuracy of the pattern reproduction. This phenomenon is particularly disadvantageous in dry etching because not only the substrate surface but also the patterned resist layer are subject to etching more or less so that the cross-sectional profile of the patterned resist layer may affect the orthogonality of the etching treatment.
The above mentioned problems could be solved, as a principle, by improving the resolving power of patterning with a decreased exposure dose so as to improve the orthogonality of the cross-sectional profile configuration of the patterned photoresist layer.
Conventional positive-working photoresist compositions are mostly of the type of a mixture comprising a photosensitive compound containing a quinone diazide group and a novolac resin as a film-forming constituent while it is known, as is taught by U.S. Pat. No. 3,148,983, a problem in a composition of this type is the poor miscibility of the quinone diazide group-containing compound with organic solvents sometimes to cause precipitation of the photosensitive compound in the photoresist composition.
As is understood from the above given description, the history of development of positive-working photoresist compositions thus far made has proceeded along the guide line to enhance the sensitivity including improvements of the miscibility of the components in the photoresist composition to prevent occurrence of precipitation therein. In this regard, it is noteworthy that the positive-working photoresist compositions conventionally available on the market are formulated mostly with a photosensitive compound which is a modification of the polyhydroxy benzophenone derivative disclosed in the above mentioned United States patent.
On the other hand, of course, efforts have been made to obtain a high contrast of the photoresist in the technology of photolithography by the minifying projection exposure for patterning relative to the manufacture of VLSIs of 256 kilobits or larger density with a fineness of 1.2 to 1.3 .mu.m or finer. The investigations therefor have been directed mainly to the improvement of the novolac resin as the film-forming constituent. In this regard, a number of reports have been published disclosing the results of the studies on the blending proportion of a cresol novolac resin [see Japanese Patent Kokai No. 62-35349 and No. 62-270951], relative positions of the methylene linkages bonding the cresol units [see Proc. of SPIF 631. Advances in Resist Technology and Processing, 1986, pages 76-82], combination of m-cresol, p-cresol and xylenol as the phenolic constituent of the novolac resin [see Proc. of SPIF 920. Advances in Resist Technology and Processing, 1988, pages 134-141] and so on.
In the above mentioned quinone diazide group-containing compound which is usually an esterification product of a polyhydroxy benzophenone and a naphthoquinone-1,2-diazide sulfonyl chloride, on the other hand, it is known that increase in the degree of the esterification reaction has an effect to improve the orthogonality of the patterned photoresist layer [see Proc. of SPIF 732. Advances in Resist Technology and Processing, 1987, pages 194-210, Japanese Patent Kokai No. 1-179147 and 1st Micro Process Conference, 1988, July, 4-6, Tokyo, pages 160-161]. The proposal for an increase in the degree of esterification, however, is not without another difficult problem because the solubility of the guinone diazide group-containing compound, which is inherently poor in the miscibility with the other ingredients in the photoresist composition, in organic solvents is further decreased by increasing the degree of esterification thereof. In this respect, various proposals and attempts have been made to select different organic solvents in which the quinone diazide group-containing compounds have good solubility even when the degree of esterification thereof is high.
Various organic solvents have been hitherto proposed as a solvent in a positive-working photoresist composition although each of them has its own advantages and disadvantages. For example, ethylene glycol monoethyl ether acetate is not good enough as a solvent for the photosensitive compound. Cyclopentanone proposed in Japanese Patent Kokai No. 59-155838 practically cannot be used as the solvent due to the eventual decomposition of the quinone diazide group-containing photosensitive compound therein resulting in a decrease in the photosensitivity of the composition though with somewhat improved solubility behavior in its part. The alkyl monohydroxycarboxylatess proposed in Japanese Patent Kokai No. 62-123444 generally have high hygroscopicity to cause eventual precipitation of the photosensitive compound when the solvent contains water by absorption so that the storage stability of the composition is greatly decreased unless in a hermetically sealed condition. Combinations of a cyclic ketone and an alcohol proposed in U.S. Pat. No. 4,526,856 also have similar disadvantages. Propylene glycol monoalkyl ether acetates proposed in Japanese Patent Kokai No. 61-7837 are impractical due to the poor miscibility thereof with the photosensitive compound.
A photoresist composition prepared by using the above named conventional organic solvents has further problems including the poor step-coverage characteristics when the composition is applied to the substrate surface having a level difference between areas making a step and eventual appearance of necking in the cross-sectional profile of the patterned resist layer formed from the composition using the solvent at the interface between the resist layer and the substrate surface not to give a satisfactory patterned resist layer.
To summarize, all of the positive-working photoresist compositions using a conventional organic solvent are defective in respect of the storage stability of the composition and the step-coverage characteristics in application to a substrate surface having a level difference. In particular, no satisfactory solvent is known for a photoresist composition containing a photosensitive compound which is an esterification product of a polyhydroxy benzophenone and naphthoquinone-1,2-diazide sulfonic acid, especially, when the degree of esterification is high, in respect of the relatively low solubility of the photosensitive compound in the solvent or the water-absorptivity of the solvent.